warrior, i had the same problem this weekend. mine were looking almost white they were so nasty. i used 3:1 diluted Zep Purple Degreaser from the Home Depot. I Scrubbed it on there with a stiff brush and let it sit. rinse, repeat. elbow grease is key here. all the grime and discoloration melted away....then i sprayed them with Meguiars hot shine tire shine...let dry...and rinse(so they aren't slippery)
DO NOT GET THIS STUFF ON ANYTHING ELSE, INCLUDING YOUR HANDS. WEAR GLOVES AND EYE PROTECTION. - seriously it's not about being a *****...the stuff will burn your eyes and i have gotten chemical burns on my hands from it in the past.